Water-Absorbing Material Having a Coating of Elastic Film-Forming Polyurethane with High Wicking and High Permeability

ABSTRACT

The invention relates to a process for producing a water-absorbing material by coating water-absorbing polymer particles with a film-forming polyurethane and silica, heat treating the coated particles and coating the heat-treated particles with pyrogenic hydrophilic silica. The invention further relates to the water-absorbing material obtainable according to the process of the invention. The water-absorbing material has improved wicking ability (FHA) and core shell saline flow conductivity (CS-SFC).

The present application relates to a water-absorbing polymer having acoating of elastic film-forming polyurethane and also to a process forits production.

An important component of disposable absorbent articles such as diapersis an absorbent core structure comprising water-absorbing polymers,typically hydrogel-forming water-absorbing polymers, also referred to asabsorbent gelling material, AGM, or super-absorbent polymers, SAPs. Thispolymer material ensures that large amounts of bodily fluids, e.g.urine, can be absorbed by the article during its use and locked away,thus providing low rewet and good skin dryness.

Especially useful water-absorbing polymers or SAPs are often made byinitially polymerizing unsaturated carboxylic acids or derivativesthereof, such as acrylic acid, alkali metal (e.g., sodium and/orpotassium) or ammonium salts of acrylic acid, alkyl acrylates, and thelike in the presence of relatively small amounts of di- orpolyfunctional monomers such as N,N′-methylenebisacrylamide,trimethylolpropane triacrylate, ethylene glycol di(meth)acrylate, ortriallylamine. The di- or polyfunctional monomer materials serve tolightly crosslink the polymer chains thereby rendering themwater-insoluble, yet water-absorbing. These lightly crosslinkedabsorbent polymers contain a multiplicity of carboxylate groups attachedto the polymer backbone. It is generally believed that the neutralizedcarboxylate groups generate an osmotic driving force for the absorptionof body fluids by the crosslinked polymer network. In addition, thepolymer particles are often treated as to form a surface cross-linkedlayer on the outer surface in order to improve their properties inparticular for application in baby diapers.

Water-absorbing (hydrogel-forming) polymers useful as absorbents inabsorbent members and articles such as disposable diapers need to haveadequately high absorption capacity, as well as adequately high gelstrength. Absorption capacity needs to be sufficiently high to enablethe absorbent polymer to absorb significant amounts of the aqueous bodyfluids encountered during use of the absorbent article. Together withother properties of the gel, gel strength relates to the tendency of theswollen polymer particles to resist deformation under an applied stress.The gel strength needs to be high enough in the absorbent member orarticle so that the particles do not deform and fill the capillary voidspaces to an unacceptable degree causing so-called gel blocking. Thisgel-blocking inhibits the rate of fluid uptake or the fluiddistribution, i.e. once gel-blocking occurs, it can substantially impedethe distribution of fluids to relatively dry zones or regions in theabsorbent article and leakage from the absorbent article can take placewell before the water-absorbing polymer particles are fully saturated orbefore the fluid can diffuse or wick past the “gel blocking” particlesinto the rest of the absorbent article. Thus, it is important that thewater-absorbing polymers (when incorporated in an absorbent structure orarticle) maintain a high wet-porosity and have a high resistance againstdeformation thus yielding high permeability for fluid transport throughthe swollen gel bed.

Surface crosslinking leads to a higher crosslinking density close to thesurface of each superabsorbent particle. This addresses the problem of“gel blocking”, which means that, with earlier types of superabsorbents,a liquid insult will cause swelling of the outermost layer of particlesof a bulk of superabsorbent particles into a practically continuous gellayer, which effectively blocks transport of further amounts of liquid(such as a second insult) to unused superabsorbent below the gel layer.While this is a desired effect in some applications of superabsorbents(for example sealing underwater cables), it leads to undesirable effectswhen occurring in personal hygiene products. Increasing the stiffness ofindividual gel particles by surface crosslinking leads to open channelsbetween the individual gel particles within the gel layer and thusfacilitates liquids transport through the gel layer. Although surfacecrosslinking decreases the CRC or other parameters describing the totalabsorption capacity of a superabsorbent sample, it may well increase thetotal amount of liquid that can be absorbed by a hygiene productcontaining a given amount of superabsorbent during normal use of theproduct.

There is still a need to provide thinner absorbent articles since theyincrease the wearing comfort. There has been a trend to remove part orall of the cellulose fibres (pulp) from the products. These ultrathinhygiene articles may comprise construction elements (for example—but notlimited to—the diaper core or the acquisition distribution layer) whichconsist of water-absorbing polymeric particles to an extent which is inthe range from 50% to 100% by weight, so that the polymeric particles inuse not only perform the storage function for the fluid but also ensureactive fluid transportation (in simple words, the capacity of a swollengel bed to pull liquid against gravity, or wicking absorption, aproperty that can be quantified as Fixed Height Absorption (“FHA”)value, determined as described below) and passive fluid transportation(in simple words, the capacity of a swollen gel bed to allow flow ofliquid with gravity, a property that can be quantified as Saline FlowConductivity (“SFC”) value, determined as described below). The greaterthe proportion of cellulose pulp which is replaced by water-absorbingpolymeric particles or synthetic fibers, the greater the number oftransportation functions which the water-absorbing polymeric particleshave to perform in addition to their storage function. It has been foundthat for such absorbent articles in particular, there is a need forwater-absorbent polymeric particles that have a good absorbent capacity(CRC value) and a good fluid transportation (reflected by a good FHAvalue and SFC value). It is well-known in the art that there is atrade-off between the wicking ability and the permeability.

WO 2009/016055 discloses water-absorbing polymeric particles with highfluid transportation and absorption performance by contacting polymerparticles with a postcrosslinker, a nitrogen-containing water-solublepolymer and a hydrophobic polymer and heat-treating the obtainedparticles.

WO 2006/082239 discloses a water-absorbing material having a coating ofelastic film-forming polymers which have high core shell centrifugeretention capacity (CS-CRC), high core shell absorbency under load(CS-AUL) and high core shell saline flow conductivity (CS-SFC).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the equipment used to measure the Saline FlowConductivity (SFC) of a swollen gel layer;

FIG. 2 illustrates the SFC apparatus L of FIG. 1 in detail; and

FIG. 3 illustrates the plunger P of FIG. 2 in detail.

It is an object of the invention to provide a water-absorbing materialhaving a high active fluid transportation rate (FHA) and a high coreshell saline flow conductivity (CS-SFC).

We have found that this object is achieved by a process of producing awater-absorbing material comprising the steps of

-   -   a) coating water-absorbing polymer particles with an aqueous        composition comprising a film-forming polyurethane;    -   b) optionally coating the polyurethane coated particles with        silica;    -   c) heat-treating the coated particles obtained in step (b) at        above 50° C.; and    -   d) coating the heat-treated particles with pyrogenic hydrophilic        silica.

The water-absorbing material herein is such that it swells in water byabsorbing the water; it may thereby form a gel. It may also absorb otherliquids and swell. Thus, when used herein, ‘water-absorbing’ means thatthe material absorbs water, and typically swells in water, but typicallyalso (in) other liquids or solutions, preferably water based liquidssuch as 0.9% saline and urine.

Inert gases within the realm of this application are materials which arein gaseous form under the respective reaction conditions and which,under these conditions, do not have an oxidizing effect on theconstituents of the reaction mixture or on the polymer, and alsomixtures of these gases. Useful inert gases include for examplenitrogen, carbon dioxide or argon, and nitrogen is preferred.

According to one embodiment of the invention, the film-formingpolyurethane and the pyrogenic silica are used in weight ratio fromabout 8:1 to about 2:1.

According to a further embodiment, the film-forming polyurethane and thepyrogenic silica are used in weight ratio from about 6:1 to about 2:1,in particular 6:1 to 3:4 and preferably 5:1 to 3:1.

Useful for the purposes of the present invention are in principle allparticulate water-absorbing polymeric particles known to one skilled inthe art from superabsorbent literature for example as described inModern Superabsorbent Polymer Technology, F. L. Buchholz, A. T. Graham,Wiley 1998. The water-absorbing polymeric particles are preferablyspherical particles of the kind typically obtained from inverse phasesuspension polymerizations; they can also be optionally agglomerated atleast to some extent to form larger irregular particles. But mostparticular preference is given to commercially available irregularlyshaped particles of the kind obtainable by current state of the artproduction processes as is more particularly described hereinbelow byway of example.

The water-absorbing polymeric particles that are to be coated accordingto the present invention are preferably polymeric particles obtainableby polymerization of a monomer solution comprising

-   i) at least one ethylenically unsaturated acid-functional monomer,-   ii) at least one crosslinker,-   iii) if appropriate one or more ethylenically and/or allylically    unsaturated monomers copolymerizable with i) and-   iv) if appropriate one or more water-soluble polymers onto which the    monomers i), ii) and    -   if appropriate iii) can be at least partially grafted, to give a        base polymer,

wherein the base polymer obtained thereby is dried, classified and ifappropriate is subsequently treated with

-   v) at least one post-crosslinker

before being dried and thermally post-crosslinked (ie. surfacecrosslinked) to give a precursor polymer.

Useful monomers i) include for example ethylenically unsaturatedcarboxylic acids, such as acrylic acid, methacrylic acid, maleic acid,fumaric acid, and itaconic acid, or derivatives thereof, such asacrylamide, methacrylamide, acrylic esters and methacrylic esters.Acrylic acid and methacrylic acid are particularly preferred monomers.Acrylic acid is most preferable.

The base polymers according to the present invention are typicallycrosslinked, i.e., the polymerization is carried out in the presence ofcompounds having two or more polymerizable groups which can befree-radically copolymerized into the polymer network. Usefulcrosslinkers ii) are disclosed in WO 2006/082239 which is incorporatedherein by reference in its entirety.

However, particularly advantageous crosslinkers ii) are di- andtriacrylates of altogether 3- to 15-tuply ethoxylated glycerol, ofaltogether 3- to 15-tuply ethoxylated trimethylolpropane, especially di-and triacrylates of altogether 3-tuply ethoxylated glycerol or ofaltogether 3-tuply ethoxylated trimethylolpropane, of 3-tuplypropoxylated glycerol, of 3-tuply propoxylated trimethylolpropane, andalso of altogether 3-tuply mixedly ethoxylated or propoxylated glycerol,of altogether 3-tuply mixedly ethoxylated or propoxylatedtrimethylolpropane, of altogether 15-tuply ethoxylated glycerol, ofaltogether 15-tuply ethoxylated trimethylolpropane, of altogether40-tuply ethoxylated glycerol and also of altogether 40-tuplyethoxylated trimethylolpropane. Where n-tuply ethoxylated means that nmols of ethylene oxide are reacted to one mole of the respective polyolwith n being an integer number larger than 0.

Very particularly preferred crosslinkers ii) are diacrylated,dimethacrylated, triacrylated or trimethacrylated multiply ethoxylatedand/or propoxylated glycerols as described for example in prior Germanpatent application DE 103 19 462.2. Di- and/or triacrylates of 3- to10-tuply ethoxylated glycerol are particularly advantageous. Veryparticular preference is given to di- or triacrylates of 1- to 5-tuplyethoxylated and/or propoxylated glycerol. The triacrylates of 3- to5-tuply ethoxylated and/or propoxylated glycerol are most preferred.These are notable for particularly low residual levels in thewater-absorbing polymer (typically below 10 ppm) and the aqueousextracts of water-absorbing polymers produced therewith have an almostunchanged surface tension compared with water at the same temperature(typically not less than 0.068 N/m).

Examples of ethylenically unsaturated monomers iii) which arecopolymerizable with the monomers i) are acrylamide, methacrylamide,crotonamide, dimethylaminoethyl methacrylate, dimethylaminoethylacrylate, dimethylaminopropyl acrylate, diethylaminopropyl acrylate,dimethylaminobutyl acrylate, dimethylaminoethyl methacrylate,diethylaminoethyl methacrylate, dimethylaminoneopentyl acrylate anddimethylaminoneopentyl methacrylate.

Useful water-soluble polymers iv) include polyvinyl alcohol,polyvinylpyrrolidone, starch, starch derivatives, polyglycols,polyacrylic acids, polyvinylamine or polyallylamine, partiallyhydrolysed polyvinylformamide or polyvinylacetamide, preferablypolyvinyl alcohol and starch.

Preference is given to base polymers which are lightly crosslinked. Thelight degree of crosslinking is reflected in a high CRC value and alsoin the fraction of extractables.

The crosslinker is preferably used (depending on its molecular weightand its exact composition) in such amounts that the base polymersproduced have a CRC between 20 and 60 g/g when their particle size isbetween 150 and 850 μm and the 16 h extractables fraction is not morethan 25% by weight. The CRC is preferably between 30 and 45 g/g, morepreferably between 33 and 40 g/g.

Particular preference is given to base polymers having a 16hextractables fraction of not more than 20% by weight, preferably notmore than 15% by weight, even more preferably not more than 10% byweight and most preferably not more than 7% by weight and whose CRCvalues are within the preferred ranges that are described above.

The preparation of a suitable base polymer and also further usefulhydrophilic ethylenically unsaturated monomers i) are described in DE-A199 41 423, EP-A 686 650, WO 01/45758 and WO 03/14300. The reaction ispreferably carried out in a kneader as described for example in WO01/38402, or on a belt reactor as described for example in EP-A-955 086.

It is further possible to use any conventional inverse suspensionpolymerization process. If appropriate, the fraction of crosslinker canbe greatly reduced or completely omitted in such an inverse suspensionpolymerization process, since self-crosslinking occurs in such processesunder certain conditions known to one skilled in the art.

It is further possible to make base polymers using any desired spraypolymerization process.

The acid groups of the base polymers obtained are preferably 30-100 mol%, more preferably 65-90 mol % and most preferably 69-85 mol %neutralized, for which the customary neutralizing agents can be used,for example ammonia, or amines, such as ethanolamine, diethanolamine,triethanolamine or dimethylaminoethanolamine, preferably alkali metalhydroxides, alkali metal oxides, alkali metal carbonates or alkali metalbicarbonates and also mixtures thereof, in which case sodium andpotassium are particularly preferred as alkali metals, but mostpreferred is sodium hydroxide, sodium carbonate or sodium bicarbonateand also mixtures thereof. Typically, neutralization is achieved byadmixing the neutralizing agent as an aqueous solution or as an aqueousdispersion or else preferably as a molten or as a solid material.

Neutralization can be carried out, after polymerization, at the basepolymer stage. But it is also possible to neutralize up to 40 mol %,preferably from 10 to 30 mol % and more preferably from 15 to 25 mol %of the acid groups before polymerization by adding a portion of theneutralizing agent to the monomer solution and to set the desired finaldegree of neutralization only after polymerization, at the base polymerstage. The monomer solution may be neutralized by admixing theneutralizing agent, either to a predetermined degree ofpreneutralization with subsequent post-neutralization to the final valueafter or during the polymerization reaction, or the monomer solution isdirectly adjusted to the final value by admixing the neutralizing agentbefore polymerization. The base polymer can be mechanically comminuted,for example by means of a meat grinder, in which case the neutralizingagent can be sprayed, sprinkled or poured on and then carefully mixedin. To this end, the gel mass obtained can be repeatedly minced forhomogenization.

The neutralized base polymer is then dried with a belt, fluidized bed,tower dryer or drum dryer until the residual moisture content ispreferably below 13% by weight, especially below 8% by weight and mostpreferably below 4% by weight, the water content being determinedaccording to EDANA's recommended test method No. 430.2-02 “Moisturecontent” (EDANA=European Disposables and Nonwovens Association). Thedried base polymer is thereafter ground and sieved, useful grindingapparatus typically include roll mills, pin mills, hammer mills, jetmills or swing mills.

The water-absorbing polymers to be used can be post-crosslinked in oneversion of the present invention. Useful post-crosslinkers v) includecompounds comprising two or more groups capable of forming covalentbonds with the carboxylate groups of the polymers. Useful compoundsinclude for example alkoxysilyl compounds, polyaziridines, polyamines,polyamidoamines, di- or polyglycidyl compounds as described in EP-A 083022, EP-A 543 303 and EP-A 937 736, polyhydric alcohols as described inDE-C 33 14 019. Useful post-crosslinkers v) are further said to includeby DE-A 40 20 780 cyclic carbonates, by DE-A 198 07 502 2-oxazolidoneand its derivatives, such as N-(2-hydroxyethyl)-2-oxazolidone, by DE-A198 07 992 bis- and poly-2-oxazolidones, by DE-A 198 54 5732-oxotetrahydro-1,3-oxazine and its derivatives, by DE-A 198 54 574N-acyl-2-oxazolidones, by DE-A 102 04 937 cyclic ureas, by German patentapplication 103 34 584.1 bicyclic amide acetals, by EP-A 1 199 327oxetanes and cyclic ureas and by WO 03/031482 morpholine-2,3-dione andits derivatives.

Post-crosslinking is typically carried out by spraying a solution of thepost-crosslinker onto the base polymer or the dry base-polymericparticles. Spraying is followed by thermal drying, and thepost-crosslinking reaction can take place not only before but alsoduring drying.

Preferred post-crosslinkers v) are amide acetals or carbamic esters ofthe general formula I

where

-   R¹ is C₁-C₁₂-alkyl, C₂-C₁₂-hydroxyalkyl, C₂-C₁₂-alkenyl or    C₆-C₁₂-aryl,-   R² is X or OR⁶,-   R³ is hydrogen, C₁-C₁₂-alkyl, C₂-C₁₂-hydroxyalkyl, C₂-C₁₂-alkenyl or    C₆-C₁₂-aryl, or X,-   R⁴ is C₁-C₁₂-alkyl, C₂-C₁₂-hydroxyalkyl, C₂-C₁₂-alkenyl or    C₆-C₁₂-aryl,-   R⁵ is hydrogen, C₁-C₁₂-alkyl, C₂-C₁₂-hydroxyalkyl, C₂-C₁₂-alkenyl,    C₁-C₁₂-acyl or C₆-C₁₂-aryl,-   R⁶ is C₁-C₁₂-alkyl, C₂-C₁₂-hydroxyalkyl, C₂-C₁₂-alkenyl, C₁-C₁₂-acyl    or C₆-C₁₂-aryl and-   X is a carbonyl oxygen common to R² and R³,

wherein R¹ and R⁴ and/or R⁵ and R⁶ can be a bridged C₂-C₆-alkanediyl andwherein the above mentioned radicals R¹ to R⁶ can still have in totalone to two free valences and can be attached through these free valencesto at least one suitable basic structure, for example 2-oxazolidones,such as 2-oxazolidone and N-hydroxyethyl-2-oxazolidone,N-hydroxypropyl-2-oxazolidone, N-methyl-2-oxazolidone,N-acyl-2-oxazolidones, such as N-acetyl-2-oxazolidone,2-oxotetrahydro-1,3-oxazine, bicyclic amide acetals, such as5-methyl-1-aza-4,6-dioxabicyclo[3.3.0]octane,1-aza-4,6-dioxabicyclo[3.3.0]octane and5-isopropyl-1-aza-4,6-dioxabicyclo[3.3.0]octane, bis-2-oxazolidones andpoly-2-oxazolidones;

or polyhydric alcohols, in which case the molecular weight of thepolyhydric alcohol is preferably less than 100 g/mol, preferably lessthan 90 g/mol, more preferably less than 80 g/mol and most preferablyless than 70 g/mol per hydroxyl group and the polyhydric alcohol has novicinal, geminal, secondary or tertiary hydroxyl groups, and polyhydricalcohols are either diols of the general formula IIa

HO—R⁶—OH   (IIa)

where R⁶ is either an unbranched dialkyl radical of the formula—(CH₂)_(m)—, where m is an integer from 3 to 20 and preferably from 3 to12, and both the hydroxyl groups are terminal, or an unbranched,branched or cyclic dialkyl radical

or polyols of the general formula IIb

where R⁷, R⁸, R⁹ and R¹⁰ are independently hydrogen, hydroxyl,hydroxymethyl, hydroxyethyloxymethyl, 1-hydroxyprop-2-yloxymethyl,2-hydroxypropyloxymethyl, methyl, ethyl, n-propyl, isopropyl, n-butyl,n-pentyl, n-hexyl, 1,2-dihydroxyethyl, 2-hydroxyethyl, 3-hydroxypropylor 4-hydroxybutyl and in total 2, 3 or 4 and preferably 2 or 3 hydroxylgroups are present, and not more than one of R⁷, R⁸, R⁹ and R¹⁰ ishydroxyl, examples being 1,3-propanediol, 1,5-pentanediol,1,6-hexanediol and 1,7-heptanediol, 1,3-butanediol, 1,8-octanediol,1,9-nonanediol and 1,10-decanediol, butane-1,2,3-triol,butane-1,2,4-triol, glycerol, trimethylolpropane, trimethylolethane,pentaerythritol, glycerol each having 1 to 3 ethylene oxide units permolecule, trimethylolethane or trimethylolpropane each having 1 to 3ethylene oxide units per molecule, propoxylated glycerol,trimethylolethane or trimethylolpropane each having 1 to 3 propyleneoxide units per molecule, 2-tuply ethoxylated or propoxylatedneopentylglycol,

or cyclic carbonates of the general formula III

where R¹¹, R¹², R¹³, R¹⁴, R¹⁵ and R¹⁶ are independently hydrogen,methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl or isobutyl, andn is either 0 or 1, examples being ethylene carbonate and propylenecarbonate,

or bisoxazolines of the general formula IV

where R¹⁷, R¹⁸, R¹⁹, R²⁰, R²¹, R²², R²³ and R²⁴ are independentlyhydrogen, methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl orisobutyl and R²⁵ is a single bond, a linear, branched or cyclicC₁-C₁₂-dialkyl radical or polyalkoxydiyl radical which is constructed ofone to ten ethylene oxide and/or propylene oxide units, and is comprisedof polyglycol dicarboxylic acids for example. An example for a compoundunder formula IV being 2,2′-bis(2-oxazoline).

The at least one post-crosslinker v) is typically used in an amount ofabout 1.50 wt. % or less, preferably not more than 0.50% by weight, morepreferably not more than 0.30% by weight and most preferably in therange from 0.001% and 0.15% by weight, all percentages being based onthe base polymer, as an aqueous solution. It is possible to use a singlepost-crosslinker v) from the above selection or any desired mixtures ofvarious post-crosslinkers.

The aqueous post-crosslinking solution, as well as the at least onepost-crosslinker v), can typically further comprise a cosolvent.Cosolvents which are technically highly useful are C₁-C₆-alcohols, suchas methanol, ethanol, n-propanol, isopropanol, n-butanol, sec-butanol,tert-butanol or 2-methyl-1-propanol, C₂-C₅-diols, such as ethyleneglycol, 1,2-propylene glycol, 1,3-propanediol or 1,4-butanediol,ketones, such as acetone, or carboxylic esters, such as ethyl acetate.

A preferred embodiment does not utilize any cosolvent. The at least onepostcrosslinker v) is then only employed as a solution in water, with orwithout an added deagglomerating aid. Deagglomerating aids are known toone skilled in the art and are described for example in DE-A-10 239 074and WO 2006/042704, which are each hereby expressly incorporated hereinby reference. Preferred deagglomerating aids are surfactants such asethoxylated and alkoxylated derivatives of 2-propylheptanol and alsosorbitan monoesters. Particularly preferred deagglomerating aids arepolyoxyethylene 20 sorbitan monolaurate and polyethylene glycol 400monostearate.

The concentration of the at least one post-crosslinker v) in the aqueouspost-crosslinking solution is for example in the range from 1% to 50% byweight, preferably in the range from 1.5% to 20% by weight and morepreferably in the range from 2% to 5% by weight, based on thepost-crosslinking solution.

In a further embodiment, the post-crosslinker is dissolved in at leastone organic solvent and spray dispensed; in this case, the water contentof the solution is less than 10 wt. %, preferably no water at all isutilized in the post-crosslinking solution.

It is, however, understood that post-crosslinkers which effectcomparable surface-crosslinking results with respect to the finalpolymer performance may of course be used in this invention even whenthe water content of the solution containing such post-crosslinker andoptionally a cosolvent is anywhere in the range of >0 to <100% byweight.

The total amount of post-crosslinking solution based on the base polymeris typically in the range from 0.3% to 15% by weight and preferably inthe range from 2% to 6% by weight. The practice of post-crosslinking iscommon knowledge to those skilled in the art and described for examplein DE-A-12 239 074 and WO 2006/042704.

Spray nozzles useful for post-crosslinking are not subject to anyrestriction. Suitable nozzles and atomizing systems are described forexample in the following literature references: Zerstäuben vonFlüssigkeiten, Expert-Verlag, volume 660, Reihe Kontakt & Studium,Thomas Richter (2004) and also in Zerstäubungstechnik, Springer-Verlag,VDI-Reihe, Günter Wozniak (2002). Mono- and polydisperse sprayingsystems can be used. Suitable polydisperse systems include one-materialpressure nozzles (forming a jet or lamellae), rotary atomizers,two-material atomizers, ultrasonic atomizers and impact nozzles.

The spraying with the solution of post-crosslinker is preferably carriedout in mixers having moving mixing implements, such as screw mixers,paddle mixers, disk mixers, plowshare mixers and shovel mixers.Particular preference is given to vertical mixers and very particularpreference to plowshare mixers and shovel mixers. Useful mixers includefor example Lödige® mixers, Bepex® mixers, Nauta® mixers, Processall®mixers and Schugi® mixers.

After spraying, the water-absorbing polymeric particles are thermallydried, and the post-crosslinking reaction can take place before, duringor after drying.

It is particularly preferable to utilize a fluidized bed dryer for thecrosslinking reaction, and the residence time is then preferably below30 minutes, more preferably below 20 minutes and most preferably below10 minutes.

To produce a very white polymer, the gas space in the dryer is kept asfree as possible of oxidizing gases; at any rate, the volume fraction ofoxygen in the gas space is not more than 14% by volume.

The water-absorbing polymeric particles can have a particle sizedistribution in the range from 45 μm to 4000 μm. Particle sizes used inthe hygiene sector preferably range from 45 μm to 1000 μm, preferablyfrom 45-850 μm, and especially from 100 μm to 850 μm. It is preferableto coat water-absorbing polymeric particles having a narrow particlesize distribution, especially 100-850 μm, or even 100-600 μm.

Narrow particle size distributions are those in which not less than 80%by weight of the particles, preferably not less than 90% by weight ofthe particles and most preferably not less than 95% by weight of theparticles are within the selected range; this fraction can be determinedusing the familiar sieve method of EDANA 420.2-02 “Particle SizeDistribution”. Selectively, optical methods can be used as well,provided these are calibrated against the accepted sieve method ofEDANA.

Preferred narrow particle size distributions have a span of not morethan 700 μm, more preferably of not more than 600 μm, and mostpreferably of less than 400 μm. Span here refers to the differencebetween the coarse sieve and the fine sieve which bound thedistribution. The coarse sieve is not coarser than 850 μm and the finesieve is not finer than 45 μm. Particle size ranges which are preferredfor the purposes of the present invention are for example fractions of150-600 μm (span: 450 μm), of 200-700 μm (span: 500 μm), of 150-500 μm(span: 350 μm), of 150-300 μm (span: 150 μm), of 300-700 μm (span: 400μm), of 400-800 μm (span: 400 μm), of 100-800 μm (span: 700 μm).

Preference is likewise given to monodisperse water-absorbing polymericparticles as obtained from the inverse suspension polymerizationprocess. It is similarly possible to select mixtures of monodisperseparticles of different diameter as water-absorbing polymeric particles,for example mixtures of monodisperse particles having a small diameterand monodisperse particles having a large diameter. It is similarlypossible to use mixtures of monodisperse with polydispersewater-absorbing polymeric particles.

Coating these water-absorbing polymeric particles having narrow particlesize distributions and preferably having a maximum particle size of ≦600μm according to the present invention provides a water-absorbingmaterial, which swells rapidly and therefore is particularly preferred.

The water-absorbing particles can be spherical in shape as well asirregularly shaped particles.

Step a)

The polyurethanes to be used according to the present invention forcoating are film forming and have elastomeric properties. They aredisclosed in WO 2006/082239 the disclosure of which is incorporatedherein in its entirety.

Film forming means that the polymer (polyurethane) can readily be madeinto a layer or coating upon evaporation of the solvent in which it isdissolved or dispersed. The polymer may for example be thermoplasticand/or crosslinked. Elastomeric means the material will exhibit stressinduced deformation that is partially or completely reversed uponremoval of the stress.

In one embodiment, the polymer has a tensile stress at break in the wetstate of at least 1 MPa, or even at least 3 MPa and more preferably atleast 5 MPa, or even at least 8 MPa. Most preferred materials havetensile stress at break in the wet state of at least 10 MPa, preferablyat least 40 MPa. This can be determined by the test method, describedbelow.

In one embodiment, particularly preferred polymers herein are materialsthat have a wet secant elastic modulus at 400% elongation(SM_(wet 400%)) of at least 0.25 MPa, preferably at least about 0.50MPa, more preferably at least about 0.75 or even at least 2.0 MPa, andmost preferably of at least about 3.0 MPa as determined by the testmethod below.

In one embodiment, preferred polymers herein have a ratio of [wet secantelastic modulus at 400% elongation (SM_(wet 400%))] to [dry secantelastic modulus at 400% elongation (SM_(dry 400%))] of 10 or less,preferably of 1.4 or less, more preferably 1.2 or less or even morepreferably 1.0 or less, and it may be preferred that this ratio is atleast 0.1, preferably at least 0.6, or even at least 0.7.

In one embodiment, the film-forming polymer is present in the form of acoating that has a shell tension, which is defined as the (theoreticalequivalent shell caliper)×(average wet secant elastic modulus at 400%elongation) of about 5 to 200 N/m, or preferably of 10 to 170 N/m, ormore preferably 20 to 130 N/m, and even more preferably 40 to 110 N/m.

In one embodiment of the invention where the water-absorbing polymerparticles herein have been surface-crosslinked (either prior toapplication of the shell described herein, or at the same time asapplying said shell), it may even be more preferred that the shelltension of the water-absorbing material is in the range from 15 N/m to60 N/m, or even more preferably from 20 N/m to 60 N/m, or preferablyfrom 40 to 60 N/m.

In yet another embodiment wherein the water absorbing polymericparticles are not surface crosslinked, it is even more preferred thatthe shell tension of the water-absorbing material is in the range fromabout 60 to 110 N/m.

In one embodiment, the film-forming polymer is present in the form of acoating on the surface of the water absorbing material, that has a shellimpact parameter, which is defined as the (average wet secant elasticmodulus at 400% elongation)*(relative Weight of the shell polymercompared to the total weight of the coated polymer) of about 0.03 MPa to0.6 MPa, preferably 0.07 MPa to 0.45 MPa, more preferably about 0.1 to0.35 MPa. The “relative weight of the shell polymer compared to thetotal weight of the coated polymer”is a fraction typically between 0.0to 1.0.

In one embodiment, preference is given to film-forming polymersespecially polyurethanes which, in contrast to the water-absorbingpolymeric particles, swell only little if at all on contact with aqueousfluids. This means in practice that the film-forming polymers havepreferably a water-swelling capacity of less than 1 g/g, or even lessthan 0.5 g/g, or even less than 0.2 g/g or even less than 0.1 g/g, asmay be determined by the method, as set out below.

In another embodiment preference is given to film-forming polymerswhich, in contrast to the water-absorbing polymeric particles, swellonly moderately on contact with aqueous fluids. This means in practicethat the film-forming polymers have preferably a water-swelling capacityof at least 1 g/g, or more than 2 g/g, or even more than 3 g/g, orpreferably 4 to 10 g/g, but less than 30 g/g, preferably less than 20g/g, most preferably less than 12 g/g, as may be determined by themethod, as set out below.

The film forming polymer is typically such that the resulting coating onthe water-swellable polymers herein is not water-soluble and, preferablynot water-dispersible.

In order to impart desirable properties to the elastic polymer,additionally fillers such as particulates, oils, solvents, plasticizers,surfactants, dispersants may be optionally incorporated.

The mechanical properties as described above are believed to becharacteristic in certain embodiments for a suitable film-formingpolymer for coating. The polymer may be hydrophobic or hydrophilic. Forfast wetting it is however preferable that the polymer is alsohydrophilic.

The film-forming polymer can for example be applied from a solution oran aqueous solution or in another embodiment can be applied from adispersion or in a preferred embodiment from an aqueous dispersion. Thesolution can be prepared using any suitable organic solvent for exampleacetone, isopropanol, tetrahydrofuran, methyl ethyl ketone, dimethylsulfoxide, dimethylformamide, chloroform, ethanol, methanol and mixturesthereof.

Polymers can also be blended prior to coating by blending theirrespective solutions or their respective dispersions. In particular,polymers that do not fulfill the elastic criteria or permeabilitycriteria by themselves can be blended with polymers that do fulfillthese criteria and yield a blend that is suitable for coating in thepresent invention.

The synthesis of polyurethanes and the preparation of polyurethanedispersions is well described for example in Ullmanns Encyclopedia ofIndustrial Chemistry, Sixth Edition, 2000 Electronic Release.

In one embodiment, the hydrophilic properties are achieved as a resultof the polyurethane comprising hydrophilic polymer blocks, for examplepolyether groups having a fraction of groups derived from ethyleneglycol (CH₂CH₂O) or from 1,4-butanediol (CH₂CH₂CH₂CH₂O) or frompropylene glycol (CH₂CH₂CH₂O), or mixtures thereof.Polyetherpolyurethanes are therefore preferred film-forming polymers.The hydrophilic blocks can be constructed in the manner of comb polymerswhere parts of the side chains or all side chains are hydrophilicpolymeric blocks. But the hydrophilic blocks can also be constituents ofthe main chain (i.e., of the polymer's backbone). A preferred embodimentutilizes polyurethanes where at least the predominant fraction of thehydrophilic polymeric blocks is present in the form of side chains. Theside chains can in turn be block copolymers such as poly(ethyleneglycol)-co-poly(propylene glycol).

It is further possible to obtain hydrophilic properties for thepolyurethanes through an elevated fraction of ionic groups, preferablycarboxylate, sulfonate, phosphonate or ammonium groups. The ammoniumgroups may be protonated or alkylated tertiary or quarternary groups.Carboxylates, sulfonates, and phosphates may be present as alkali-metalor ammonium salts. Suitable ionic groups and their respective precursorsare for example described in “Ullmanns Encyclopädie der technischenChemie”, 4^(th) Edition, Volume 19, p. 311-313 and are furthermoredescribed in DE-A 1 495 745 and WO 03/050156.

It is well understood by those skilled in the art that polyurethanesalso include allophanate, biuret, carbodiimide, oxazolidinyl,isocyanurate, uretdione, and other linkages in addition to urethane andurea linkages.

In one embodiment the block copolymers useful herein are preferablypolyether urethanes and polyester urethanes. Especially preferred arepolyether urethanes comprising polyalkylene glycol units, especiallypolyethylene glycol units or poly(tetramethylene glycol) units.

As used herein, the term “alkylene glycol” includes both alkyleneglycols and substituted alkylene glycols having 2 to 10 carbon atoms,such as ethylene glycol, 1,3-propylene glycol, 1,2-propylene glycol,1,2-butylene glycol, 1,3-butylene glycol, 1,4-butylene glycol, styreneglycol and the like.

The polyurethanes used according to the present invention are generallyobtained by reaction of polyisocyanates with active hydrogen-containingcompounds having two or more reactive groups. These include

-   a) high molecular weight compounds having a molecular weight in the    range of preferably 300 to 100 000 g/mol especially from 500 to 30    000 g/mol-   b) low molecular weight compounds and-   c) compounds having polyether groups, especially polyethylene oxide    groups or polytetrahydrofuran groups and a molecular weight in the    range from 200 to 20 000 g/mol, the polyether groups in turn having    no reactive groups.

These compounds can also be used as mixtures.

Suitable polyisocyanates have an average of about two or more isocyanategroups, preferably an average of about two to about four isocyanategroups and include aliphatic, cycloaliphatic, araliphatic, and aromaticpolyisocyanates, used alone or in mixtures of two or more. Diisocyanatesare more preferred. Especially preferred are aliphatic andcycloaliphatic polyisocyanates, especially diisocyanates.

Specific examples of suitable aliphatic diisocyanates include alpha,omega-alkylene diisocyanates having from 5 to 20 carbon atoms, such ashexamethylene-1,6-diisocyanate, 1,12-dodecane diisocyanate,2,2,4-trimethylhexamethylene diisocyanate, 2,4,4-trimethyl-hexamethylenediisocyanate, 2-methyl-1,5-pentamethylene diisocyanate, and the like.Polyisocyanates having fewer than 5 carbon atoms can be used but areless preferred because of their high volatility and toxicity. Preferredaliphatic polyisocyanates include hexamethylene-1,6-diisocyanate,2,2,4-trimethylhexamethylene diisocyanate, and2,4,4-trimethyl-hexamethylene diisocyanate.

Specific examples of suitable cycloaliphatic diisocyanates includedicyclohexylmethane diisocyanate, (commercially available as Desmodur® Wfrom Bayer Corporation), isophorone diisocyanate, 1,4-cyclohexanediisocyanate, 1,3-bis(isocyanatomethyl)cyclohexane, and the like.Preferred cycloaliphatic diisocyanates include dicyclohexylmethanediisocyanate and isophorone diisocyanate.

Specific examples of suitable araliphatic diisocyanates includem-tetramethyl xylylene diisocyanate, p-tetramethyl xylylenediisocyanate, 1,4-xylylene diisocyanate, 1,3-xylylene diisocyanate, andthe like. A preferred araliphatic diisocyanate is tetramethyl xylylenediisocyanate.

Examples of suitable aromatic diisocyanates include 4,4′-diphenylmethanediisocyanate, toluene diisocyanate, their isomers, naphthalenediisocyanate, and the like. A preferred aromatic diisocyanate is toluenediisocyanate and 4,4′-diphenylmethane diisocyanate.

Examples of high molecular weight compounds a) having 2 or more reactivegroups are such as polyester polyols and polyether polyols, as well aspolyhydroxy polyester amides, hydroxyl-containing polycaprolactones,hydroxyl-containing acrylic copolymers, hydroxyl-containing epoxides,polyhydroxy polycarbonates, polyhydroxy polyacetals, polyhydroxypolythioethers, polysiloxane polyols, ethoxylated polysiloxane polyols,polybutadiene polyols and hydrogenated polybutadiene polyols,polyacrylate polyols, halogenated polyesters and polyethers, and thelike, and mixtures thereof. The polyester polyols, polyether polyols,polycarbonate polyols, polysiloxane polyols, and ethoxylatedpolysiloxane polyols are preferred. Particular preference is given topolyesterpolyols, polycarbonate polyols and polyalkylene ether polyolsand in particular to polyesterpolyols. The number of functional groupsin the aforementioned high molecular weight compounds is preferably onaverage in the range from 1.8 to 3 and especially in the range from 2 to2.2 functional groups per molecule.

The polyester polyols typically are esterification products prepared bythe reaction of organic polycarboxylic acids or their anhydrides with astoichiometric excess of a diol. The diols used in making the polyesterpolyols include alkylene glycols, e.g., ethylene glycol, 1,2- and1,3-propylene glycols, 1,2-, 1,3-, 1,4-, and 2,3-butane diols, hexanediols, neopentyl glycol, 1,6-hexanediol, 1,8-octanediol, and otherglycols such as bisphenol-A, cyclohexanediol, cyclohexane dimethanol(1,4-bis-hydroxymethylcycohexane), 2-methyl-1,3-propanediol,2,2,4-trimethyl-1,3-pentanediol, diethylene glycol, triethylene glycol,tetraethylene glycol, polyethylene glycol, dipropylene glycol,polypropylene glycol, dibutylene glycol, polybutylene glycol, dimeratediol, hydroxylated bisphenols, polyether glycols, halogenated diols, andthe like, and mixtures thereof. Preferred diols include ethylene glycol,diethylene glycol, butane diol, hexane diol, and neopentylglycol.Alternatively or in addition, the equivalent mercapto compounds may alsobe used.

Suitable carboxylic acids used in making the polyester polyols includedicarboxylic acids and tricarboxylic acids and anhydrides, e.g., maleicacid, maleic anhydride, succinic acid, glutaric acid, glutaricanhydride, adipic acid, suberic acid, pimelic acid, azelaic acid,sebacic acid, chlorendic acid, 1,2,4-butane-tricarboxylic acid,o-phthalic acid, the isomers of phthalic acid, phthalic anhydride,fumaric acid, dimeric fatty acids made from oleic acid, and the like,and mixtures thereof. Preferred polycarboxylic acids used in making thepolyester polyols include aliphatic or aromatic dibasic acids.

Examples of suitable polyester polyols include poly(glycol adipate)s,poly(ethylene terephthalate) polyols, polycaprolactone polyols,orthophthalic polyols, sulfonated and phosphonated polyols, and thelike, and mixtures thereof.

The preferred polyester polyol is a diol. Preferred polyester diolsinclude poly(butanediol adipate); hexanediol adipic acid and isophthalicacid polyesters such as hexaneadipate isophthalate polyester; hexanediolneopentyl glycol adipic acid polyester diols, e.g., Piothane 67-3000 HNA(Panolam Industries) and Piothane 67-1000 HNA, as well as propyleneglycol maleic anhydride adipic acid polyester diols, e.g., PiothaneSO-1000 PMA, and hexane diol neopentyl glycol fumaric acid polyesterdiols, e.g., Piothane 67-SO0 HNF. Other preferred Polyester diolsinclude Rucoflex®S101.5-3.5, S1040-3.5, and S-1040-110 (BayerCorporation).

Polyether polyols are obtained in known manner by the reaction of astarting compound that contain reactive hydrogen atoms, such as water orthe diols set forth for preparing the polyester polyols, and alkyleneglycols or cyclic ethers, such as ethylene glycol, propylene glycol,butylene glycol, styrene glycol, ethylene oxide, propylene oxide,1,2-butylene oxide, 2,3-butylene oxide, oxetane, tetrahydrofuran,epichlorohydrin, and the like, and mixtures thereof. Preferredpolyethers include poly(ethylene glycol), poly(propylene glycol),polytetrahydrofuran, and co [poly(ethylene glycol)-poly(propyleneglycol)]. Polyethylenglycol and Polypropyleneglycol can be used as suchor as physical blends. In case that propyleneoxide and ethylenoxide arecopolymerized, these polypropylene-co-polyethylene polymers can be usedas random polymers or block-copolymers.

In one embodiment the polyetherpolyol is a constituent of the mainpolymer chain.

In another embodiment the polyetherol is a terminal group of the mainpolymer chain. In yet another embodiment the polyetherpolyol is aconstituent of a side chain which is comb-like attached to the mainchain. An example of such a monomer is Tegomer D-3403 (Degussa).

Polycarbonates include those obtained from the reaction of diols such1,3-propanediol, 1,4-butanediol, 1,6-hexanediol, diethylene glycol,triethylene glycol, tetraethylene glycol, and the like, and mixturesthereof with dialkyl carbonates such as diethyl carbonate, diarylcarbonates such as diphenyl carbonate or phosgene.

Examples of low molecular weight compounds b) having two reactivefunctional groups are the diols such as alkylene glycols and other diolsmentioned above in connection with the preparation of polyesterpolyols.They also include diamines such as diamines and polyamines which areamong the preferred compounds useful in preparing the aforesaidpolyesteramides and polyamides. Suitable diamines and polyamines include1,2-diaminoethane, 1,6-diaminohexane, 2-methyl-1,5-pentanediamine,2,2,4-trimethyl-1,6-hexanediamine, 1,12-diaminododecane, 2-aminoethanol,2-[(2-aminoethyl)amino]-ethanol, piperazine, 2,5-dimethylpiperazine,1-amino-3-aminomethyl-3,5,5-trimethylcyclohexane (isophorone diamine orIPDA), bis-(4-aminocyclohexyl)-methane,bis-(4-amino-3-methyl-cyclohexyl)-methane, 1,4-diaminocyclohexane,1,2-propylenediamine, hydrazine, urea, amino acid hydrazides, hydrazidesof semicarbazidocarboxylic acids, bis-hydrazides and bis-semicarbazides,diethylene triamine, triethylene tetramine, tetraethylene pentamine,pentaethylene hexamine, N,N,N-tris-(2-aminoethyl)amine,N-(2-piperazinoethyl)-ethylene diamine,N,N′-bis-(2-aminoethyl)-piperazine, N,N,N′-tris-(2-aminoethyl)ethylenediamine, N4N-(2-aminoethyl)-2-aminoethyl]-N′-(2-aminoethyl)-piperazine,N-(2-aminoethyl)-N′-(2-piperazinoethyl)-ethylene diamine,N,N-bis-(2-aminoethyl)-N-(2-piperazinoethyl)amine,N,N-bis-(2-piperazinoethyl)amine, polyethylene imines,iminobispropylamine, guanidine, melamine, N-(2-aminoethyl)-1,3-propanediamine, 3,3′-diaminobenzidine, 2,4,6-triaminopyrimidine,polyoxypropylene amines, tetrapropylenepentamine, tripropylenetetramine,N,N-bis-(6-aminohexyl)amine, N,N′-bis-(3-aminopropyl)ethylene diamine,and 2,4-bis-(4′-aminobenzyl)-aniline, and the like, and mixturesthereof. Preferred diamines and polyamines include1-amino-3-aminomethyl-3,5,5-trimethyl-cyclohexane (isophorone diamine orIPDA), bis-(4-aminocyclohexyl)-methane,bis-(4-amino-3-methylcyclohexyl)-methane, ethylene diamine, diethylenetriamine, triethylene tetramine, tetraethylene pentamine, andpentaethylene hexamine, and the like, and mixtures thereof. Othersuitable diamines and polyamines for example include Jeffamine® D-2000and D-4000, which are amine-terminated polypropylene glycols differingonly by molecular weight, and Jeffamine® XTJ-502, T 403, T 5000, and T3000 which are amine terminated polyethyleneglycols, amine terminatedco-polypropylenepolyethylene glycols, and triamines based onpropoxylated glycerol or trimethylolpropane and which are available fromHuntsman Chemical Company.

The polyurethane may additionally contain functional groups which canundergo further crosslinking reactions and which can optionally renderthem self-crosslinkable.

Compounds having at least one additional crosslinkable functional groupinclude those having carboxylic, carbonyl, amine, hydroxyl, andhydrazide groups, and the like, and mixtures of such groups. The typicalamount of such optional compound is up to about 1 milliequivalent,preferably from about 0.05 to about 0.5 milliequivalent, and morepreferably from about 0.1 to about 0.3 milliequivalent per gram of finalpolyurethane on a dry weight basis.

The preferred compounds for incorporation of carboxylic groups into theisocyanate-terminated prepolymer are hydroxy-carboxylic acids having thegeneral formula (HO)_(x)Q(COOH)_(y) wherein Q is a straight or branchedhydrocarbon radical having 1 to 12 carbon atoms, and x and y are 1 to 3.Examples of such hydroxy-carboxylic acids include citric acid,dimethylolpropanoic acid (DMPA), dimethylol butanoic acid (DMBA),glycolic acid, lactic acid, malic acid, dihydroxymaleic acid, tartaricacid, hydroxypivalic acid, and the like, and mixtures thereof.Dihydroxy-carboxylic acids are more preferred with dimethylolpropanoicacid (DMPA) being most preferred.

Other suitable compounds providing crosslinkability include thioglycolicacid, 2,6-dihydroxybenzoic acid, and the like, and mixtures thereof.

Optional neutralization of the prepolymer having pendant carboxyl groupsconverts the carboxyl groups to carboxylate anions, thus having awater-dispersibility enhancing effect. Suitable neutralizing agentsinclude tertiary amines, metal hydroxides, ammonia, and other agentswell known to those skilled in the art.

As a chain extender, at least one of water, an inorganic or organicpolyamine having an average of about 2 or more primary and/or secondaryamine groups, polyalcohols, ureas, or combinations thereof is suitablefor use in the present invention. Suitable organic amines for use as achain extender include diethylene triamine (DETA), ethylene diamine(EDA), meta-xylylenediamine (MXDA), aminoethyl ethanolamine (AEEA),2-methyl pentane diamine, and the like, and mixtures thereof. Alsosuitable for practice in the present invention are propylene diamine,butylene diamine, hexamethylene diamine, cyclohexylene diamine,phenylene diamine, tolylene diamine, 3,3-dichlorobenzidene,4,4′-methylene-bis-(2-chloroaniline), 3,3-dichloro-4,4-diaminodiphenylmethane, sulfonated primary and/or secondary amines, and thelike, and mixtures thereof. Suitable inorganic and organic aminesinclude hydrazine, substituted hydrazines, and hydrazine reactionproducts, and the like, and mixtures thereof. Suitable polyalcoholsinclude those having from 2 to 12 carbon atoms, preferably from 2 to 8carbon atoms, such as ethylene glycol, diethylene glycol, neopentylglycol, butanediols, hexanediol, and the like, and mixtures thereof.Suitable ureas include urea and its derivatives, and the like, andmixtures thereof. Hydrazine is preferred and is most preferably used asa solution in water. The amount of chain extender typically ranges fromabout 0.5 to about 0.95 equivalents based on available isocyanate.

Particularly suitable elastic film-forming polyurethanes are extensivelydescribed in the literature references hereinbelow and expressly formpart of the subject matter of the present disclosure. Particularlyhydrophilic thermoplastic polyurethanes are sold by Noveon, Brecksville,Ohio, under the tradenames of Permax® 120, Permax 200 and Permax 220 andare described in detail in “Proceedings International Waterborne HighSolids Coatings, 32, 299, 2004” and were presented to the public inFebruary 2004 at the “International Waterborne, High-Solids, and PowderCoatings Symposium” in New Orleans, USA. The preparation is described indetail in US 2003/0195293. Furthermore, the polyurethanes described inU.S. Pat. No. 4,190,566, U.S. Pat. No. 4,092,286, US 2004/0214937 andalso WO 03/050156 expressly form part of the subject matter of thepresent disclosure.

More particularly, the polyurethanes described can be used in mixtureswith each other or with other film-forming polymers, fillers, oils,water-soluble polymers or plasticizing agents in order that particularlyadvantageous properties may be achieved with regard to hydrophilicity,water perviousness and mechanical properties.

The polymeric film is preferably applied in an amount of 0.1-25 parts byweight of the film-forming polymer (reckoned as solids material) to 100parts by weight of dry water-absorbing polymeric particles. The amountof film-forming polymer used per 100 parts by weight of water-absorbingpolymeric particles is preferably 0.1-15 parts by weight, especially0.5-10 parts by weight, more preferably 0.5-7 parts by weight, even morepreferably 0.5-5 parts by weight and in particular 0.5-4.5 parts byweight, 0.5-4 parts by weight or 0.5-3 parts by weight with an amount of0.5-2.5 parts by weight and in particular 1-2 parts by weight beingparticularly preferred.

The film-forming polyurethane can be applied as a solid material, as ahotmelt, as a dispersion, as an aqueous dispersion, as an aqueoussolution or as an organic solution to the particles of thewater-absorbing addition polymer. The form in which the film-formingpolyurethane is applied to the water-absorbing polymeric particles ispreferably as a solution or more preferably as an aqueous dispersion.

Suitable elastomeric polymers which are applicable from solution are forexample Estane® 58245 (Noveon, Cleveland, USA), Estane® 4988, Estane®4986, Estane® X-1007, Estane® T5410, Irogran® PS370-201 (HuntsmanPolyurethanes), Irogran® VP 654/5, or Elastollan® LP 9109 (Elastogran).

Useful solvents for polyurethanes include solvents which make itpossible to establish 1 to not less than 40% by weight concentrations ofthe polyurethane in the respective solvent or mixture. As examples theremay be mentioned alcohols, esters, ethers, ketones, amides, andhalogenated hydrocarbons like methyl ethyl ketone, acetone, isopropanol,tetrahydrofuran, dimethylformamide, N-alkylpyrrolidones such asN-methylpyrrolidone or N-ethylpyrrolidone, chloroform and mixturesthereof. Solvents which are polar, aprotic and boil below 100° C. atambient pressure are particularly advantageous.

In a preferred embodiment the polyurethane is in the form of an aqueousdispersion. Preferred aqueous polyurethane dispersions are Hauthane®HD-4638 (ex Hauthaway), Hydrolar® HO 269 (ex Colm, Italy), Impraperm®48180 (ex Bayer Material Science AG, Germany), Lurapret® DPS (ex BASFGermany), Permax®120, Permax® 200, and Permax® 220 (ex Noveon,Brecksville, Ohio),), Syntegra®YM2000 and Syntegra®YM2100 (ex Dow,Midland, Mich.) Witcobond G-213, Witcobond G-506, Witcobond G-507,Witcobond® 736 (ex Uniroyal Chemical, Middlebury, Conn.) and Astacin®Finish products, in particular Astacin® Finish PUMN TF (ex BASFGermany).

Aqueous herein refers to water and also mixtures of water with up to 20%by weight of water-miscible solvents, based on the total amount ofsolvent. Water-miscible solvents are miscible with water in the desireduse amount at 25° C. and 1 bar. They include alcohols such as methanol,ethanol, propanol, isopropanol, ethylene glycol, 1,2-propanediol,1,3-propanediol, ethylene carbonate, glycerol and methoxyethanol andwater-soluble ethers such as tetrahydrofuran and dioxane.

It is particularly preferable to effect the coating in a fluidized bedreactor. The water-absorbing particles are introduced as generallycustomary, depending on the type of the reactor, and are generallycoated by spraying with the film-forming polymer as a solid material orpreferably as a polymeric solution or dispersion. Aqueous dispersions ofthe film-forming polymer are particularly preferred for this.

Useful fluidized bed reactors include for example the fluidized orsuspended bed coaters familiar in the pharmaceutical industry.Particular preference is given to the Wurster process and theGlatt-Zeller process and these are described for example in“Pharmazeutische Technologie, Georg Thieme Verlag, 2nd edition (1989),pages 412-413” and also in “Arzneiformenlehre, WissenschaftlicheVerlagsbuchandlung mbH, Stuttgart 1985, pages 130-132”. Particularlysuitable batch and continuous fluidized bed processes on a commercialscale are described in Drying Technology, 20(2), 419-447 (2002).

The process of the present invention utilizes the aforementionednozzles, which are customarily used for post-crosslinking. However,two-material nozzles are particularly preferred.

It is possible that the water-absorbing material comprises two or morelayers of coating agent (shells), obtainable by coating thewater-absorbing polymers twice or more. This may be the same coatingagent or a different coating agent. However, preference for economicreasons is given to a single coating.

According to the present invention, coating takes place at a productand/or carrier gas temperature in the range from 0° C. to 50° C.,preferably at 5-45° C., especially 10-40° C. and most preferably 15-35°C.

Step b)

After the film-forming polyurethane has been applied and before thesubsequent heat-treating step the particles are optionally coated withsilica. Amorphous or crystalline forms of silicon dioxide are suitableand they can also be used in their hydrated forms. Silica is preferablyused as aqueous commercially available silica sol, such as for exampleLevasil® Kieselsole (H.C. Starck GmbH), which have particle sizes in therange 5-75 nm.

In a preferred embodiment, the silica is used in an amount in the rangefrom 0.001% to 20% by weight, preferably less than 10% by weight,especially in the range from 0.001% to 5% by weight, more preferably inthe range from 0.001% to 2% by weight and most preferably between 0.001and 1% by weight, based on the weight of the water-absorbingpolyurethane.

In addition to the silica it is also possible to add knowndeagglomerating aids, examples being waxes and preferably micronized orpreferably partially oxidized polyethylenic waxes, which can likewise beused in the form of an aqueous dispersion. Such waxes are described inEP 0 755 964, which is hereby expressly incorporated herein byreference.

According to an embodiment of the invention a surfactant can be used instep (a) and/or (b). The addition of the surfactant can take placetogether with the polyurethane, before the addition of the polyurethane,after the addition of the polyurethane or together with the silica instep (b). The surfactant can further be applied during thepost-crosslinking operation.

Useful surfactants include nonionic, anionic and cationic surfactantsand also mixtures thereof. Preferred are nonionic surfactants. Usefulnonionic surfactants include for example sorbitan esters, such as themono-, di- or triesters of sorbitans with C₈-C₁₈-carboxylic acids suchas lauric, palmitic, stearic and oleic acids; polysorbates;alkylpolyglucosides having 8 to 22 and preferably 10 to 18 carbon atomsin the alkyl chain and 1 to 20 and preferably 1.1 to 5 glucoside units;N-alkylglucamides; alkylamine alkoxylates or alkylamide ethoxylates;alkoxylated C₈-C₂₂-alcohols such as fatty alcohol alkoxylates or oxoalcohol alkoxylates; block polymers of ethylene oxide, propylene oxideand/or butylene oxide; alkylphenol ethoxylates having C₆-C₁₄-alkylchains and 5 to 30 mol of ethylene oxide units.

The amount of surfactant is generally in the range from 0.01% to 0.5% byweight, preferably less than 0.1% by weight and especially below 0.05%by weight, based on the weight of the water-absorbing material.

Step c)

According to the invention, heat-treating takes place at temperaturesabove 50° C., preferably in a temperature range from 100 to 200° C.,especially 120-160° C. Without wishing to be bound by theory, theheat-treating causes the applied film-forming polymer, preferablypolyurethane, to flow and form a polymeric film whereby the polymerchains are entangled. The duration of the heat-treating is dependent onthe heat-treating temperature chosen and the glass transition andmelting temperatures of the film-forming polymer. In general, aheat-treating time in the range from 30 minutes to 120 minutes will befound to be sufficient. However, the desired formation of the polymericfilm can also be achieved when heat-treating for less than 30 minutes,for example in a fluidized bed dryer. Longer times are possible, ofcourse, but especially at higher temperatures can lead to damage in thepolymeric film or to the water-absorbing material.

The heat-treating is carried out for example in a downstream fluidizedbed dryer, a tunnel dryer, a tray dryer, a tower dryer, one or moreheated screws or a disk dryer or a Nara® dryer. Heat-treating ispreferably done in a fluidized bed reactor and more preferably directlyin the Wurster Coater.

In one embodiment for the process steps of coating, heat-treating, andcooling, it may be possible to use air or dried air in each of thesesteps.

In other embodiments an inert gas may be used in one or more of theseprocess steps. In yet another embodiment one can use mixtures of air andinert gas in one or more of these process steps.

The heat-treating is preferably carried out under inert gas. It isparticularly preferable that the coating step be carried out under inertgas as well. It is very particularly preferable when the concludingcooling phase is carried out under protective gas too. Preference istherefore given to a process where the production of the water-absorbingmaterial according to the present invention takes place under inert gas.

After the heat-treating step has been concluded, the driedwater-absorbing polymeric materials are cooled. To this end, the warmand dry polymer is preferably continuously transferred into a downstreamcooler. This can be for example a disk cooler, a Nara paddle cooler or ascrew cooler.

Step d)

After the heat-treating step the particles are coated with pyrogenichydrophilic silica in powder form. Pyrogenic silica is well-known in theart, see for example Angew. Chem., 1960, 744-750. The primary particleshave a diameter of about 10 to about 40 nm. If prepared according to theconventional process of flame hydrolysis without post-treatment,hydrophilic pyrogenic silica is obtained. Pyrogenic hydrophilic silicais commercially available under the name Aerosil®, in particularAerosil® 90, Aerosil® 130, Aerosil® 150, Aerosil® 200, Aerosil® 300,Aerosil® 380, Aerosil® OX 50, Aerosil® EG 50, Aerosil® TT 600, Aerosil®200 SP, Aerosil® 300 SP, Sipernat®, in particular Sipernat® 22S, orAcematt® TS 100.

The hydrophilic pyrogenic silica is in general used in an amount of 0.05to 8% by weight, in particular in an amount of 0.1 to 5% by weight, andpreferably in an amount of 0.5 to 3% by weight or 0.5 to 2% by weight,based on the weight of the polyurethane.

The coating is in general carried out in conventional mixers, such asSchuggi mixers, turbolizers, tumbler mixers or plowshare mixers such asa Loedige kneader which can be used alone or preferably as a battery ofplural consecutive units. If such a mixer is used alone, the particlesto be coated may have to be fed multiple times through the apparatus tobecome homogeneously coated. If two or more of such apparatus are set upas consecutive units then one pass may be sufficient.

The invention also relates to the water-absorbing material which isobtainable according to the process of the invention.

The coated water-absorbing particles may be present in thewater-absorbing material of the invention mixed with other particlescomponents, such as fibers, (fibrous) glues, organic or inorganic fillermaterials or flowing aids, process aids, anti-caking agents, odorcontrol agents, coloring agents, coatings to impart wet stickiness,hydrophilic surface coatings, etc.

The water-absorbing material is typically obtainable by the processdescribed herein, which is such that the resulting material is solid;this includes gels, flakes, fibers, agglomerates, large blocks,granules, particles, spheres and other forms known in the art for thewater-absorbing polymers described hereinafter.

The water-absorbing material of the invention preferably comprises lessthan 20% by weight of water, or even less than 10% or even less than 8%or even less than 5%, or even no water. The water content of thewater-absorbing material can be determined by the Edana test, number ERT430.1-99 (February 1999) which involves drying the water-absorbingmaterial at 105° Celsius for 3 hours and determining the moisturecontent by the weight loss of the water-absorbing materials afterdrying.

The invention further relates to a water-absorbing polymer materialwhich has a FHA of at least 10 g/g, preferably of at least 12 g/g and inparticular of at least 14 g/g. The water-absorbing polymer materialfurther has a CS-SFC of at least 200·10⁻⁷ cm³ s/g, preferably at least230·10⁻⁷ cm³ s/g, more preferably at least 250·10⁻⁷ cm³ s/g and inparticular at least 280·10⁻⁷ cm³ s/g.

According to another embodiment the water-absorbable polymer materialwhich has a FHA of at least 10 g/g and a CS-SFC of at least 200·10⁻⁷ cm³s/g is obtainable according to the process of the invention.

The water-absorbing material of the present invention is notable for thefact that the particles, which have an irregular shape when dry, assumein the swollen state a more rounded shape/morphology, since the swellingof the absorbent core is distributed via the rebound forces of theelastic polymeric envelope over the surface and the elastic polymericenvelope substantially retains its properties in this respect during theswelling process and in use. The enveloping film-forming polyurethane ispermeable to saline, so that the polymer particles achieve excellentabsorption values in the CS-CRC (Core Shell Centrifugation RetentionCapacity) test and also good permeability in the CS-SFC test.

In addition, the water-absorbing materials of the invention have a highpermeability for liquid flow through the gel bed as can be measured withthe CS-SFC test set out herein.

The water-absorbing material, hereinafter also referred to ashydrogel-forming polymer, was tested by the test methods describedherein below.

Methods:

The measurements should be carried out, unless otherwise stated, at anambient temperature of 23±2° C. and a relative humidity of 50±10%. Thewater-absorbing polymeric particles are thoroughly mixed through beforemeasurement. For the purpose of the following methods AGM means“Absorbent Gelling Material” and can relate to the water absorbingpolymer particles as well as to the water-absorbing material. Therespective meaning is clearly defined by the data given in the examplesbelow. The test methods, for example for determining CRC, CS-CRC, AUL,CS-AUL, SFC, and CCRC are disclosed in WO 2006/082239 and areincorporated by reference. Measurement of FHA and CS-FSC is carried outas described below.

Saline Flow Conductivity (SFC)

The method to determine the permeability of a swollen gel layer is the“Saline Flow Conductivity” also known as “Gel Layer Permeability” and isdescribed in EP A 640 330. The equipment used for this method has beenmodified as described below. FIG. 1 shows the permeability measurementequipment set-up with the open-ended tube for air admittance A,stoppered vent for refilling B, constant hydrostatic head reservoir C,Lab Jack D, delivery tube E, stopcock F, ring stand support G, receivingvessel H, balance I and the SFC apparatus L.

FIG. 2 shows the SFC apparatus L consisting of the metal weight M, theplunger shaft N, the lid O, the center plunger P and the cylinder Q.

The cylinder Q has an inner diameter of 6.00 cm (area=28.27 cm²). Thebottom of the cylinder Q is faced with a stainless-steel screen cloth(mesh width: 0.036 mm; wire diameter: 0.028 mm) that is bi-axiallystretched to tautness prior to attachment. The plunger consists of aplunger shaft N of 21.15 mm diameter. The upper 26.0 mm having adiameter of 15.8 mm, forming a collar, a perforated center plunger Pwhich is also screened with a stretched stainless-steel screen (meshwidth: 0.036 mm; wire diameter: 0.028 mm), and annular stainless steelweights M. The annular stainless steel weights M have a center bore sothey can slip on to plunger shaft and rest on the collar. The combinedweight of the center plunger P, shaft and stainless-steel weights M mustbe 596 g (±6 g), which corresponds to 0.30 PSI over the area of thecylinder. The cylinder lid O has an opening in the center for verticallyaligning the plunger shaft N and a second opening near the edge forintroducing fluid from the reservoir into the cylinder Q.

The cylinder Q specification details are:

Outer diameter of the Cylinder: 70.35 mm

Inner diameter of the Cylinder: 60.0 mm

Height of the Cylinder: 60.5 mm

The cylinder lid O specification details are:

Outer diameter of SFC Lid: 76.05 mm

Inner diameter of SFC Lid: 70.5 mm

Total outer height of SFC Lid: 12.7 mm

Height of SFC Lid without collar: 6.35 mm

Diameter of hole for Plunger shaft positioned in the center: 22.25 mm

Diameter of hole in SFC lid: 12.7 mm

Distance centers of above mentioned two holes: 23.5 mm

The metal weight M specification details are:

Diameter of Plunger shaft for metal weight: 16.0 mm

Diameter of metal weight: 50.0 mm

Height of metal weight: 39.0 mm

FIG. 3 shows the plunger center P specification details

Diameter m of SFC Plunger center: 59.7 mm

Height n of SFC Plunger center: 16.5 mm

14 holes o with 9.65 mm diameter equally spaced on a 47.8 mm bolt circleand 7 holes p with a diameter of 9.65 mm equally spaced on a 26.7 mmbolt circle ⅝ inches thread q.

Prior to use, the stainless steel screens of SFC apparatus, should beaccurately inspected for clogging, holes or over stretching and replacedwhen necessary. An SFC apparatus with damaged screen can delivererroneous SFC results, and must not be used until the screen has beenfully replaced.

Measure and clearly mark, with a permanent fine marker, the cylinder ata height of 5.00 cm (±0.05 cm) above the screen attached to the bottomof the cylinder. This marks the fluid level to be maintained during theanalysis. Maintenance of correct and constant fluid level (hydrostaticpressure) is critical for measurement accuracy.

A constant hydrostatic head reservoir C is used to deliver NaCl solutionto the cylinder and maintain the level of solution at a height of 5.0 cmabove the screen attached to the bottom of the cylinder. The bottom endof the reservoir air-intake tube A is positioned so as to maintain thefluid level in the cylinder at the required 5.0 cm height during themeasurement, i.e., the height of the bottom of the air tube A from thebench top is the same as the height from the bench top of the 5.0 cmmark on the cylinder as it sits on the support screen above thereceiving vessel. Proper height alignment of the air intake tube A andthe 5.0 cm fluid height mark on the cylinder is critical to theanalysis. A suitable reservoir consists of a jar containing: ahorizontally oriented L-shaped delivery tube E for fluid delivering, anopen-ended vertical tube A for admitting air at a fixed height withinthe reservoir, and a stoppered vent B for re-filling the reservoir. Thedelivery tube E, positioned near the bottom of the reservoir C, containsa stopcock F for starting/stopping the delivery of fluid. The outlet ofthe tube is dimensioned to be inserted through the opening in thecylinder lid 0, with its end positioned below the surface of the fluidin the cylinder (after the 5 cm height is attained). The air-intake tubeis held in place with an o-ring collar. The reservoir can be positionedon a laboratory jack D in order to adjust its height relative to that ofthe cylinder. The components of the reservoir are sized so as to rapidlyfill the cylinder to the required height (i.e., hydrostatic head) andmaintain this height for the duration of the measurement. The reservoirmust be capable to deliver liquid at a flow rate of minimum 3 g/sec forat least 10 minutes.

Position the plunger/cylinder apparatus on a ring stand with a 16 meshrigid stainless steel support screen (or equivalent). This supportscreen is sufficiently permeable so as to not impede fluid flow andrigid enough to support the stainless steel mesh cloth preventingstretching. The support screen should be flat and level to avoid tiltingthe cylinder apparatus during the test. Collect the fluid passingthrough the screen in a collection reservoir, positioned below (but notsupporting) the support screen. The collection reservoir is positionedon a balance accurate to at least 0.01 g. The digital output of thebalance is connected to a computerized data acquisition system.

Preparation of Reagents

Following preparations are referred to a standard 1 liter volume. Forpreparation multiple than 1 liter, all the ingredients must becalculated as appropriate.

Javco Synthetic Urine

Fill a 1 L volumetric flask with de-ionized water to 80% of its volume,add a stir bar and put it on a stirring plate. Separately, using aweighing paper or beaker weigh (accurate to ±0.01 g) the amounts of thefollowing dry ingredients using the analytical balance and add them intothe volumetric flask in the same order as listed below. Mix until allthe solids are dissolved then remove the stir bar and dilute to 1 Lvolume with distilled water. Add a stir bar again and mix on a stirringplate for a few minutes more. The conductivity of the prepared solutionmust be 7.6±0.23 mS/cm.

Chemical Formula Anhydrous Hydrated

Potassium Chloride (KCl) 2.00 g

Sodium Sulfate (Na₂SO₄) 2.00 g

Ammonium dihydrogen phosphate (NH₄H₂PO₄) 0.85 g

Ammonium phosphate, dibasic ((NH₄)₂HPO₄) 0.15 g

Calcium Chloride (CaCl₂) 0.19 g (2 H₂O) 0.25 g

Magnesium chloride (MgCl₂) 0.23 g (6 H₂O) 0.50 g

To make the preparation faster, wait until total dissolution of eachsalt before adding the next one. Jayco may be stored in a clean glasscontainer for 2 weeks. Do not use if solution becomes cloudy. Shelf lifein a clean plastic container is 10 days.

0.118 M Sodium Chloride (NaCl) Solution

Using a weighing paper or beaker weigh (accurate to ±0.01 g) 6.90 g ofsodium chloride into a 1 L volumetric flask and fill to volume withde-ionized water. Add a stir bar and mix on a stirring plate until allthe solids are dissolved. The conductivity of the prepared solution mustbe 12.50±0.38 mS/cm.

Test Preparation

Using a reference metal cylinder (40 mm diameter; 140 mm height) set thecaliper gauge (e.g. Mitotoyo Digimatic Height Gage) to read zero. Thisoperation is conveniently performed on a smooth and level bench top.Position the SFC apparatus without AGM under the caliper gauge andrecord the caliper as L₁ to the nearest of 0.01 mm.

Fill the constant hydrostatic head reservoir with the 0.118 M NaClsolution. Position the bottom of the reservoir air-intake tube A so asto maintain the top part of the liquid meniscus in the SFC cylinder atthe required 5.0 cm height during the measurement. Proper heightalignment of the air-intake tube A at the 5 cm fluid height mark on thecylinder is critical to the analysis.

Saturate an 8 cm fritted disc (7 mm thick; e.g. Chemglass Inc. #CG201-51, coarse porosity) by adding excess synthetic urine on the top ofthe disc. Repeating until the disc is saturated. Place the saturatedfritted disc in the hydrating dish and add the synthetic urine until itreaches the level of the disc. The fluid height must not exceed theheight of the disc.

Place the collection reservoir on the balance and connect the digitaloutput of the balance to a computerized data acquisition system.Position the ring stand with a 16 mesh rigid stainless steel supportscreen above the collection dish. This 16 mesh screen should besufficiently rigid to support the SFC apparatus during the measurement.The support screen must be flat and level.

AGM Sampling

AGM samples should be stored in a closed bottle and kept in a constant,low humidity environment. Mix the sample to evenly distribute particlesizes. Remove a representative sample of material to be tested from thecenter of the container using the spatula. The use of a sample divideris recommended to increase the homogeneity of the sample particle sizedistribution.

SFC Procedure

Position the weighing funnel on the analytical balance plate and zerothe balance. Using a spatula weigh 0.9 g (±0.05g) of AGM into theweighing funnel. Position the SFC cylinder on the bench, take theweighing funnel and gently, tapping with finger, transfer the AGM intothe cylinder being sure to have an evenly dispersion of it on thescreen. During the AGM transfer, gradually rotate the cylinder tofacilitate the dispersion and get homogeneous distribution. It isimportant to have an even distribution of particles on the screen toobtain the highest precision result. At the end of the distribution theAGM material must not adhere to the cylinder walls. Insert the plungershaft into the lid central hole then insert the plunger center into thecylinder for few centimeters. Keeping the plunger center away from AGMinsert the lid in the cylinder and carefully rotate it until thealignment between the two is reached. Carefully rotate the plunger toreach the alignment with lid then move it down allowing it to rest ontop of the dry AGM. Insert the stainless steel weight to the plunger rodand check if the lid moves freely. Proper seating of the lid preventsbinding and assures an even distribution of the weight on the gel bed.

The thin screen on the cylinder bottom is easily stretched. To preventstretching, apply a sideways pressure on the plunger rod, just above thelid, with the index finger while grasping the cylinder portion of theapparatus. This “locks” the plunger in place against the inside of thecylinder so that the apparatus can be lifted. Place the entire apparatuson the fritted disc in the hydrating dish. The fluid level in the dishshould not exceed the height of the fritted disc. Care should be takenso that the layer does not loose fluid or take in air during thisprocedure. The fluid available in the dish should be enough for all theswelling phase. If needed, add more fluid to the dish during thehydration period to ensure there is sufficient synthetic urineavailable. After a period of 60 minutes, place the SFC apparatus underthe caliper gauge and record the caliper as L₂ to the nearest of 0.01mm. Calculate, by difference L₂−L₁, the thickness of the gel layer as L₀to the nearest ±0.1 mm. If the reading changes with time, record onlythe initial value.

Transfer the SFC apparatus to the support screen above the collectiondish. Be sure, when lifting the apparatus, to lock the plunger in placeagainst the inside of the cylinder. Position the constant hydrostatichead reservoir such that the delivery tube is placed through the hole inthe cylinder lid. Initiate the measurement in the following sequence:

a) Open the stopcock of the constant hydrostatic head reservoir andpermit the fluid to reach the 5 cm mark. This fluid level should beobtained within 10 seconds of opening the stopcock.

b) Once 5 cm of fluid is attained, immediately initiate the datacollection program.

With the aid of a computer attached to the balance, record the quantityof fluid passing through the gel layer versus time at intervals of 20seconds for a time period of 10 minutes. At the end of 10 minutes, closethe stopcock on the reservoir. The data from 60 seconds to the end ofthe experiment are used in the calculation. The data collected prior to60 seconds are not included in the calculation. Perform the test intriplicate for each AGM sample.

Evaluation of the measurement remains unchanged from EP-A 640 330.Through-flux is captured automatically.

Saline flow conductivity (SFC) is calculated as follows:

SFC [cm³ s/g]=(Fg(t=0)×L ₀)/(d×A×WP),

where Fg(t=0) is the through-flux of NaCl solution in g/s, which isobtained from a linear regression analysis of the Fg(t) data of thethrough-flux determinations by extrapolation to t=0, L₀ is the thicknessof the gel layer in cm, d is the density of the NaCl solution in g/cm³,A is the area of the gel layer in cm² and WP is the hydrostatic pressureabove the gel layer in dyn/cm².

CS-SFC (Core Shell Saline Flow Conductivity)

CS-SFC is determined completely analogously to SFC, with the followingchanges:

To modify the SFC the person skilled in the art will design the feedline including the stopcock in such a way that the hydrodynamicresistance of the feed line is so low that prior to the start of themeasurement time actually used for the evaluation an identicalhydrodynamic pressure as in the SFC (5 cm) is attained and is also keptconstant over the duration of the measurement time used for theevaluation.

-   -   the weight of AGM used is 1.50+/−0.05 g    -   a 0.9% by weight sodium chloride solution is used as solution to        preswell the AGM sample and for through-flux measurement    -   the preswell time of the sample for measurement is 240 minutes    -   for preswelling, a filter paper 90 mm in diameter (Schleicher &        Schüll, No 597) is placed in a 500 ml crystallizing dish        (Schott, diameter=115 mm, height=65 mm) and 250 ml of 0.9% by        weight sodium chloride solution are added, then the SFC        measuring cell with the sample is placed on the filter paper and        swelling is allowed for 240 minutes    -   the through-flux data are recorded every 5 seconds, for a total        of 3 minutes    -   the points measured between 10 seconds and 180 seconds are used        for evaluation and Fg(t=0) is the through-flux of NaCl solution        in g/s which is obtained from a linear regression analysis of        the Fg(t) data of the through-flux determinations by        extrapolation to t=0    -   the stock reservoir bottle in the SFC-measuring apparatus for        through-flux solution contains about 5 kg of sodium chloride        solution.

Fixed Height Absorption (FHA)

The FHA is a method to determine the ability of a swollen gel layer totransport fluid by wicking. It is executed and evaluated as described onpage 9 and 10 in EP 01 493 453 A1.

The following adjustments need to be made versus this description:

Laboratory conditions are 23±2° C. and relative humidity is no more than50%.

Glass frit: 500 ml glass frit P40, as defined by ISO 4793, nominal poresize 16-40 μm, thickness 7 mm, e.g. Duran Schott pore size class 3. At20° C.: a 30 cm diameter disk must be capable of a water flow of 50ml/min for a pressure drop of 50 mbar.

Flexible plastic Tygon tube, for connecting the separatory funnel withthe funnel with frit. Length must be sufficient to allow for 20 cmvertical movement of the funnel.

Use of high wet strength cellulose tissue, maximum basis weight 24.6g/cm², size 80×80 mm, minimum wet tensile strength 0.32 N/cm (CDdirection), and 0.8 N/cm (MD direction), e.g. supplied by FripaPapierfabrik Albert Friedrich KG, D-63883 Miltenberg.

The tissue is clamped with a metal ring on the bottom side of the sampleholder.

Calculation:

FHA=(m3−m2)÷(m2−m1)

weight of absorbed saline solution per 1 g of AGM,

with

m1=weight of empty sample holder in g,

m2=weight of sample holder with dry AGM in g,

m3=weight of sample holder with wet AGM in g.

FHA is only determined in the context of the present invention with ahydrostatic column pressure corresponding to FHA at 20 cm.

The following examples illustrate the invention without limiting it.

EXAMPLE 1

Synthesis of the Base Polymer

A Loedige VT 5R-MK plowshare kneader of 5 l capacity was charged with206.5 g of deionized water, 271.6 g of acrylic acid, 2115.6 g of 37.3wt.-% sodium acrylate solution (100 mole % neutralized) and 3.5 g of athreefold ethoxylated glycerol triacrylate crosslinker and inertized bybubbling nitrogen through it for 20 minutes. This was followed by theaddition of dilute aqueous solutions of 2.453 g of sodium persulfate(dissolved in 13.9 g of water), 0.053 g of ascorbic acid (dissolved in10.46 g of water) and also 0.146 g of 30% by weight hydrogen peroxide(dissolved in 1.31 g of water) to initiate the polymerization at about20° C. After initiation, the temperature of the heating jacket wascontrolled in order to monitor the temperature inside the reactor. Thetemperature was kept below 90° C. The obtained crumbly gel was dried ina circulating air drying cabinet at 160° C. for about 3 hours. The driedbase polymer was subjected to milling and classified to 150 to 710 μm bysieving off over- and undersize particles.

EXAMPLE 2

Post-Crosslinking

A post-crosslinking solution was used which contained inwater/isopropanol (69.1/30.9):

0.14 wt.-% of a solution (50 wt.-%) of 2-hydroxyethyl oxazolidinone in1,3-propanediol 0.7 wt.-% 1,2-propanediol

50 ppm (based on polymer) of sorbitan monooleate (2 wt.-% solution inwater)

A Loedige VT 5R-MK plowshare kneader was charged with 1200 g of the basepolymer and heated to a product temperature of 185° C. Subsequently,4.55 wt.-% (based on polymer) of the post-crosslinking solution wassprayed on the polymer by means of nitrogen (0.5 bar) at 185° C. and arotating speed of 200 rpm using a 2-fluid nozzle. After thespray-coating the mixing was continued for 40 minutes. The obtainedproduct was classified to 710 μm and is designated as precursor.

EXAMPLE 3 Reference Example

Coating of the Precursor

In a cylindric fluidized bed (diameter of 150 mm having a sparger platewith 2 mm orifices) 1200 g of the precursor were coated with a mixtureof 66 g of 40 wt.-% polyurethane dispersion (Permax 200; LubrizolCorp.), 0.66 g Pluronic PE 6200 and 74.3 g of water at 35° C.Subsequently, the coated precursor was coated in the same fluidized bedunder the same conditions with a mixture of 12 g Levasil 50/50 (50 wt.-%of a silica dispersion) and 18 g of water. A Loedige M5 plowshare mixerM5 was then charged with 1200 g of the obtained coated precursor andthen heated to 180° C. product temperature and kept for 45 minutes. Theobtained product is the reference product.

After cooling the following parameters were determined:

FHA: 4.6 g/g

CS-SFC: 268·10⁻⁷ cm³ s/g

EXAMPLE 4

Coating of the Reference Product

The reference product of example 3 was coated in a tumbler mixer with 1%by weight each of Aerosil 200 (hydrophilic silica), Aerosil 300(hydrophilic silica) or Aerosil R 812 or Aerosil R 805 (hydrophobicsilica). The following FHAs were obtained:

a) without coating FHA 4.6 g/g; CS-SFC 268 cm³s/g (reference example 3):b) coating with Aerosil 200 FHA 12.8 g/g;  CS-SFC 265 cm³s/g(invention): c) coating with Aerosil 300 FHA 12.1 g/g;   CS-SFC 221cm³s/g (invention): d) coating with Aerosil FHA 2.6 g/g; R 812(comparison): e) coating with Aerosil FHA 3.2 g/g; CS-SFC 401 cm³s/g R805 (comparison):

EXAMPLE 5 Reference Example

Coating of the Precursor

The same fluidized bed as used in example 3 was charged with 2000 gprecursor (classified to 150-400 μm) and coated with a mixture of 78.9 gof a 37 wt.-% polyurethane dispersion (Astacin® Finish PUMN TF) and 71.1g of water at 35° C. Subsequently, the coated precursor was coated inthe same fluidized bed and under the same conditions with a mixture of20 g Levasil 50/50 (50 wt.-% of a silica dispersion) and 30 g of water.

Subsequently, a Loedige M5 plowshare mixer M5 was charged with 1200 g ofthe obtained coated product and heated to a product temperature of 180°C. and kept for 25 minutes. The obtained product is the referenceproduct. After cooling FHA was determined:

FHA: 4.6 g/g

CS-SFC: 500·10⁻⁷ cm³ s/g

EXAMPLE 6

Coating of the Reference Product

The reference product of example 5 was coated in a tumbler mixer with 1%by weight of Aerosil 200 (hydrophilic silica) and 1% by weight ofAerodisp W 7520N (aqueous dispersion of hydrophilic silica). Thefollowing FHAs were obtained:

a) without coating (reference example 5): FHA 4.63.9 g/g;   CS-SFC: 500· 10⁻⁷ cm³s/g; b) coating with Aerosil 200 (according to invention): FHA16.4 g/g; CS-SFC: 310 · 10⁻⁷ cm³s/g; c) coating with Aerodisp W 7520N(comparative example): FHA  2.6 g/g; CS-SFC: 293 · 10⁻⁷ cm³s/g; d)coating with Aerosil 200 dispersed in water (13.7 g Aerosil in 123 gwater): FHA 15.3 g/g; CS-SFC: 303 · 10⁻⁷ cm³s/g;

1. A process for producing a water-absorbing material comprising thesteps of a) coating water-absorbing polymer particles with an aqueouscomposition comprising a film-forming polyurethane; b) optionallycoating the polyurethane coated particles with a silica; c)heat-treating the coated particles obtained in step (b) at above 50° C.;and d) coating the heat-treated particles with a pyrogenic hydrophilicsilica.
 2. The process of claim 1, wherein the silica in step (b) isused as an aqueous dispersion.
 3. The process of claim 1, wherein thefilm-forming polyurethane and the silica in step (b) are used in aweight ratio from about 8:1 to about 2:1.
 4. The process of claim 3,wherein the film-forming polyurethane and the silica in step (b) areused in a weight ratio from about 6:1 to about 3:1.
 5. The process ofclaim 1, wherein the film-forming polyurethane is used as an aqueousdispersion.
 6. The process of claim 1, wherein the polyurethane on thebasis of polyester polyols is used.
 7. The process of claim 1, whereinthe water-absorbing polymer particles are spray-coated in one or more ofsteps (a), (b), and (d).
 8. The process of claim 7, wherein thespray-coating is carried out at a temperature from about 0° C. to about50° C.
 9. The process of claim 7, wherein the coating is applied in aWurster coater or in a Glatt-Zeller coater or in a continuous fluidizedbed reactor or in a continuous spouted bed reactor.
 10. The process ofclaim 9, wherein a gas stream in the fluidized bed reactor is selectedsuch that a relative moisture at a point of exit of the gas stream is inthe range from about 10% to about 90%.
 11. The process of claim 1,wherein the heat-treating is carried out in a continuous fluidized bed.12. The process of claim 1, wherein the heat-treating is carried out ata temperature in the range from about 100° C. to about 200° C.
 13. Theprocess of claim 1, wherein the heat-treating step and, optionally oneor more of the coating steps are carried out under inert gas.
 14. Theprocess of claim 1, wherein the pyrogenic hydrophilic silica is used inan amount of 0.1 to 5% by weight, based on the weight of thepolyurethane.
 15. The process of claim 1, wherein the pyrogenichydrophilic silica is used in powder form or in the form of an aqueousdispersion.
 16. The process of claim 1, which further comprises the stepof obtaining the water-absorbing polymer particles by polymerizing atleast one ethylenically unsaturated acid-functional monomer and at leastone crosslinker.
 17. The process of claim 16, wherein the monomer isacrylic acid.
 18. The process of claim 16, wherein the crosslinker is anacrylate ester of a polyalcohol.
 19. The process of claim 18, whereinthe crosslinker is a triacrylate of ethoxylated glycerine.
 20. Theprocess of claim 1, wherein the water-absorbing polymer particles aresurface post-crosslinked.
 21. The process of claim 20, wherein thepost-crosslinker is 2-oxazolidinone orN-(2-hydroxyethyl)2-oxazolidinone.
 22. A water-absorbing polymermaterial prepared by a process according to claim
 1. 23. Awater-absorbing polymer material having a FHA of at least 10 g/g and aCS-SFC of at least 200·10⁻⁷ cm³ s/g.
 24. The water-absorbing polymermaterial of claim 23 having a FHA of at least 12 g/g.